The original engine for the Austin Cooper trim level was the 997cc Austin A-series. The Cooper S was 1071 in this time period - this car was not an 'S'. These engines were practically the Model T of their day in England - in everything from Spitfires to MGs and dozens of boring passenger sedans, about 11 million A-series engines were built.
But only in the Mini is the transmission unit fixed to the bottom of the sump, in one of history's first mass-market transverse installations.
The current engine is a 2013 build by Jemal Ketat at http://cooperroadmini.com and Tony at Minimania. Have not dyno'd it yet. The goal was torque and reliability as my daily driver. It's a high-performance street build, probably in the top 10% among US-based minis.
Starting with the SU HiF44 carb, the standard reliable fast setup, you don't have to sync multiple smaller carbs this way. Just needle it and forget it. This feeds into a polished minispares intake manifold to the dual intake ports. My favorite mod is the aluminum 9-stud head from Pierce manifolds in San Jose, with 1.4" intake valves, unleaded seats and Isky springs. Attached with ARP studs.
The cam is Minimania 103, their classic fast/rally torque cam, not terribly lumpy but these cars idle pretty high, aroud 1000rpm. It is not a peaky setup but really pulls strong from 2500-4000.
The short block was skimmed .10" to get the compression to 9.5. Bored over .30" to go from 1275 to 1330cc. Russell 3-ring pistons from Australia. The bottom end is already very tough so journals were honed and that's it. A lighter A-series crank replaces the heavy A Austin America crank.
The Lucas 25D distributor has the Pertronix ignitor upgrade with a Flamethrower coil and Ultrik silicon wires.
The exhaust is the class Maniflow RC40 with an LCB header (hard to fit) and Maniflow's new megaphone front pipe unit, a street adaption of their race megaphone. This feeds into a center-exit "DTM" maniflow rear box with two-pipe exit.